An electric double layer capacitor operates on the principle that an electricity is stored by allowing a physisorption of anion and cation included in an electrolyte, on positive-pole and negative-pole surfaces of polarizable electrodes.
For example, a bipolar-type electric double layer capacitor (hereinafter referred to as capacitor) 100 includes a plurality of cells 104 each of which includes an ion-permeable separator 101 and activated-carbon electrodes 102 and 103 disposed respectively at both sides of the separator 101, as shown in FIG. 4. The capacitor 100 is formed by providing a charge collector 105 between the adjacent cells 104 and by layering or stacking the plurality of cells 104. The activated-carbon electrodes 102 and 103 are impregnated with an electrolytic solution. A packing member 106 is disposed at an outer peripheral portion of the cell 104 so as not to leak this electrolytic solution. Moreover, this packing member 106 has a function to provide electrical isolation between the stacked cells 104.
A module includes cells 104 having the number corresponding to a necessary withstand voltage (withstand voltage of single cell is about equal to 2.5 V), packing members 106, and charge collectors 105, each of which are stacked. The capacitor 100 is constructed by fastening both surfaces of the module respectively to end plates 109 and 111 through collector electrodes 107 and 108. Accordingly, the capacitor 100 maintains its enclosed structure.
In the above-mentioned capacitor 100, the activated-carbon electrodes 102 and 103, the separators 101, and the collector electrodes 107 and 108 are connected with lead wires (not shown) which are connected in series inside the module so as to allow the capacitor 100 to have a withstand voltage corresponding to (single-cell withstand voltage)×(the number of stacked layers). Such a stacked capacitor unit has an advantage that cables and the like are unnecessary as compared with a general winding-type capacitor having the same capacitance, and therefore can be designed compactly to have a high withstand voltage. Accordingly, an installation area thereof can be reduced.
The above-mentioned capacitor 100 is produced by alternately stacking the activated-carbon electrodes 102 and 103, the separator 101, the charge collector 105, the packing member 106 and the like; by sealing the portions between respective cells 104; and then by putting the electrolytic solution into the inside of the capacitor so that the activated-carbon electrodes 102 and 103 and the separator 101 are impregnated with the electrolytic solution. Specifically, the electrolytic solution is introduced into an electrolytic-solution inlet 110 provided in one spot of the end plate 109, and then is made to diffuse into all the cells 104 through a hole 107a formed in the collector electrode 107 and a hole 105a formed in the charge collector 105.
As the above-mentioned electrolytic solution, an organic-solvent-base electrolytic solution and the like can be cited. In a capacitor using this organic-solvent-base electrolytic solution, it is necessary that a moisture is prevented from mixing with the electrolytic solution existing in the capacitor in order to suppress a characteristic degradation and a gas generation, by preventing an entry of the moisture. Therefore, as shown in FIG. 4, a film 114 which is a lamination of a metal aluminum foil, a polypropylene, a polyethylene or the like (hereinafter, referred to as an aluminum laminate film) encloses the whole of capacitor 100 to seal the capacitor 100 by welding an entrance portion of the film 114. However, the entry of moisture occurs at a slight pinhole or at a portion of welding failure having a size greater than a gas molecule level. Hence, the above-mentioned characteristic degradation becomes obvious with a long-term use. Accordingly, it is necessary to achieve a moisture blocking more certainly in order to maintain the characteristic of capacitor for a long time.
For the purpose of detecting the presence or absence of the pinhole and/or the welding failure in the aluminum laminate film 114 provided for the moisture blocking, for example, there is a method in which an inside of the film and the inside of the capacitor are depressurized to vacuum state and then are sealed. By bringing the inside of the film to the vacuum state by means of such a method, an external pressure sticks the film to the capacitor. In the case where the pinhole exists in the film, the vacuum state of the inside cannot be maintained so that the film which has been pressed by the external pressure is made not to receive this external pressure. Thereby, in this case, the film becomes loose enough to easily confirm the looseness from its appearance. Also in the case where the welding failure of gas-molecule-level size exists in the film, the looseness of the film is similarly caused after leaving the film for several hours or several days. Thereby, the presence of the welding failure is determined. Moreover in these cases, a size of the pinhole and a diameter of the welding failure can be estimated based on a time duration for causing such looseness (an elapsed time from a time point when the inside became in the vacuum state). This elapsed time is used as an important inspection parameter for determining welding requirements. If such a failure is detected in this inspection, a reprocessing is conducted.
Patent Document 1: Japanese Patent Application Publication No. 2002-313677